Over the past decade a pursuit of solid state ultra fast scaleable devices, such as magnetic sensors of nanosize proportions, based on both the charge and spin of an electron has led to a development of new fields of magnetoelectronics and spintronics. The discovery of giant magnetoresistance (GMR) in magnetic multilayers has quickly led to important applications in storage technology. GMR is a phenomenon where a relatively small change in magnetism results in a large change in the resistance of the devices.
The phenomenon of a large tunnel magnetoresistance (TMR) of ferromagnet-insulator-ferromagnet structures is a focus of product development teams in many leading companies. TMR is typically observed in F1-I-F2 structures made of two ferromagnetic layers, F1 and F2, of similar or different materials separated by the insulating thin tunnel barrier 1 with thickness typically ranging between 1.4–2 nm. The tunnel current through the structure may differ significantly depending on whether the magnetic moments are parallel (low resistance) or anti parallel (high resistance). For example, in ferromagnets such as Ni80Fe20, Co—Fe, and the like, resistance may differ by up to 50% at room temperature for parallel (low resistance) versus antiparallel (high resistance) moments on ferromagnetic electrodes.
It is worth mentioning recent studies of the giant ballistic magnetoresistance of Ni nanocontacts. Ballistic magnetoresistance is observed in Ni and some other nanowires where the typical cross-section the nanocontacts of the nanowire is a few square nanometers. The transport in this case is through very short constriction and it is believed to be with conservation of electron momentum (ballistic transport). The change in the contact resistance can be close to 10 fold (or about 1000%).
All magnetic sensors proposed and developed to present day, including the read heads, are based on variations of magnetic configurations, domain structures, in ferromagnets under externally applied magnetic fields. This mechanism does not ensure operating speed and sensitivity required of ultra fast sensors.
Interest has been particularly been keen on the injection of spin-polarized carriers, mainly in the form of spin-polarized electrons into semiconductors. This is large due to relatively large spin-coherence lifetime of electrons in semiconductors, possibilities for use in ultra fast devices. One such device in the works is an ultra fast and sensitive magnetic sensor such as a magnetic read head.
The possibility of spin injection from ferromagnetic semiconductors (FMS) into nonmagnetic semiconductors has been demonstrated in a number of recent publications. However, the Curie temperature (the temperature above which a material becomes non-magnetic) of magnetic semiconductors is substantially below room temperature. The low Curie temperature limits possible applications. Room-temperature spin injection from ferromagnets (FM) into semiconductors also has been demonstrated, but it remains a difficult task and the efficiency is very low. The low spin injection efficiency makes it very difficult, if not impossible, to develop an ultra fast magnetic sensor operable at room temperature.